1. Field of the Invention
The present invention relates to a weighing device such as person-weighing scales, food-weighing scales, or any other platform weigher.
2. Description of the Prior Art
In French patent application No. 82 20040 of Dec. 2, 1982, there has already been described a weighing appliance comprising a base plate and a platform which is substantially parallel to the base plate and designed to carry the load to be weighed. Between the platform and the base plate is placed a metal bar of square or circular cross-section, the opposite ends of which are rigidly fixed to said base plate and to said platform in order to permit flexural deformation of the bar under the action of the load applied on the platform.
The metal bar carries four strain gages, the axes of which are parallel to the axis of said bar and which are electrically connected in a Wheatstone bridge circuit.
Two of the adjacent strain gages in the Wheatstone bridge are located on one face of the bar (for example the face that is nearest the platform) and the two other gages are located on the opposite face.
This arrangement of the strain gages makes it possible to measure at the output terminals of the Wheatstone bridge a signal which is theoretically proportional to the force applied on the weighing platform.
In view of the fact that the resultant of the forces applied on the platform may be located at a point remote from the axis of flexure of the bar, the torsional stresses to which the bar may be subjected are liable to produce measurement errors which can attain 5 to 6%.
In the device described in the cited French patent application No. 82 20040, the drawback just mentioned has been suppressed by connecting two torsion-sensitive strain gages having axes inclined at 45.degree. to the axis of the bar, in series with the electric supply terminals of the Wheatstone bridge formed by the four flexure-sensitive strain gages and by connecting an adjustable resistor in parallel with the two gages aforesaid and one of the terminals for measurement of the signal delivered by the Wheatstone bridge.
This arrangement makes it possible to cancel the torsional stresses detected by the flexure-sensitive strain gages.
However, the arrangement of the four flexure-sensitive strain gages on two opposite faces of the bar has the effect of introducing a considerable complication in the mass production of the weighing device and in the calibration of the device.
The precise object of the invention is to overcome the disadvantage just mentioned.